Internal Structure of the Heart

Left and Right Atria

The right atrium is the receiving chamber for deoxygenated blood returning from systemic circulation and the heart tissues. A large vein called the superior vena cava drains blood from the head, neck, upper limbs, and thoracic regions. A second large vein, the inferior vena cava, drains blood from the lower limbs and abdominopelvic regions. The left atrium receives oxygenated blood from the right and left lungs via four pulmonary veins.

Right and Left Ventricles

Blood from the right atrium enters the right ventricle. When the right ventricle contracts, it ejects deoxygenated blood into the pulmonary trunk artery. The blood then travels to the lungs through the left and right pulmonary arteries that branch from the pulmonary trunk artery. The left ventricle receives oxygenated blood from the right atrium and pumps it into the systemic circuit through the aorta artery.

Heart Valves

Blood flows through the heart due to pressure differences, moving from high-pressure to low-pressure areas. The heart valves regulate blood flow and ensure that it moves in the correct direction.

Atrioventricular Valves

The right atrioventricular (tricuspid) valve regulates blood flow between the left atrium and left ventricle. The left atrioventricular valve (bicuspid or mitrial valve) controls blood flow between the left atrium and left ventricle. The flaps of the atrioventricular valves attach to strands of connective tissue called the chordae tendineae. These resilient, thin tendinous cords connect each flap to papillary muscles, which protrude from the inferior ventricles. The chordae tendineae anchor the valve flaps so they cannot open into the atrium during ventricular contraction, thus preventing the backflow of blood into the atria.

Semilunar Valves

The pulmonary semilunar (pulmonic) valve regulates blood flow between the right ventricle and the pulmonary trunk artery. When the ventricle relaxes, the chamber pressure drops, and blood flows back toward the ventricle from the pulmonary trunk. The reverse blood flow causes the valves to close, preventing blood from entering the right ventricle. The aortic semilunar (aortic) valve regulates blood movement between the aorta and left ventricle. When the left ventricle relaxes, the ventricular pressure drops and the backflow of blood closes the valve, stopping blood from flowing into the chamber.